Dual-Phase Extraction

The following description of dual-phase extraction is an excerpt from Chapter XI of OUST's publication: How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers. (EPA 510-B-95-007). This publication also describes 9 additional alternative technologies for remediation of petroleum releases. You can download PDF files of every chapter of the document at: http://www.epa.gov/swerust1/pubs/tums.htm.

Dual-phase extraction (DPE), also known as
multi-phase extraction, vacuum-enhanced
extraction, or sometimes bioslurping, is an in-situ
technology that uses pumps to remove
various combinations of contaminated
groundwater, separate-phase petroleum
product, and hydrocarbon vapor from the
subsurface. Extracted liquids and vapor are
treated and collected for disposal, or re-injected to the subsurface (where
permissible under applicable state laws).

Application

DPE systems can be effective in removing separate-phase
product (free product) from the subsurface, thereby
reducing concentrations of petroleum
hydrocarbons in both the saturated and
unsaturated zones of the subsurface. DPE
systems are typically designed to maximize extraction rates;
however, the technology also stimulates
biodegradation of petroleum constituents in
the unsaturated zone by increasing the
supply of oxygen, in a manner similar to that of
bioventing.

DPE is often selected
because it enhances groundwater and/or
product recovery rates, especially in layered,
fine-grained soils. The application of DPE
also maximizes the effectiveness of
soil vapor extraction (SVE) by
lowering the water table and therefore
increasing air-phase permeabilities in the
vadose zone.

Operation Principles

The vacuum applied to the subsurface
with DPE systems creates vapor-phase
pressure gradients toward the vacuum well.
These vapor-phase pressure gradients are
also transmitted directly to the subsurface
liquids present, and those liquids existing in a
continuous phase (e.g., water and "free"
petroleum product) will flow toward the
vacuum well in response to the imposed
gradients (the term "free" product is a
commonly used, though imprecise term
because a greater fraction of resident
petroleum product may be recovered using
vacuum-enhanced DPE compared to the
fraction of product recoverable using gravity
drainage alone). The higher the applied
vacuum, the larger the hydraulic gradients
that can be achieved in both vapor and liquid
phases, and thus the greater the vapor and
liquid recovery rates.

Dramatic enhancements in both water and
petroleum product recovery rates resulting
from the large hydraulic gradients attainable
with DPE systems have been reported in the
literature (Blake and Gates, 1986; Blake, et al.,
1990; Bruce, et al., 1992). The depressed
groundwater table that results from these
high recovery rates serves both to
hydraulically control groundwater migration
and to increase the efficiency of vapor
extraction. The remedial effectiveness of DPE
within the zone of dewatering that commonly
develops during DPE application should be
greater than that of air sparging
due to the more uniform air flow developed
using DPE (Johnson, et al., 1992).

System Design

Although this general class of
technologies is broadly referred to as dual-phase extraction,
significant variations in the technology exist.
DPE systems often apply relatively high
vacuums to the subsurface. Thus, the
adjective "high-vacuum" is sometimes used
to describe DPE technologies, even though
all DPE systems are not high-vacuum
systems. DPE technologies can be divided
into two general categories:

subsurface liquid(s) and soil vapor
are extracted together as a high-velocity dual-phase (liquid(s) and vapor) stream using a
single pump, or

subsurface liquid(s) and soil vapor are extracted
separately using two or more pumps.

Single-Pump Systems. Single-pump systems rely on high-velocity airflow to lift suspended liquid
droplets upwards by frictional drag through
an extraction tube to the land surface. Single-pump vacuum extraction systems can be
used to extract groundwater or combinations
of separate-phase product and groundwater.

Single-pump DPE systems represent a recent
adaptation of the long-established
technology known as "vacuum groundwater extraction". This
technology has been used for many decades as a standard method
for extracting groundwater to control seepage
or effect dewatering during excavation, construction and
mining activities.

Dual-Pump Systems. The somewhat more conventional dual-pump
systems use one pump to extract liquids from
the well and a surface blower (the second
pump) to extract soil vapor. A third DPE
configuration uses a total of three pumps,
including the surface blower together with
one pump to extract floating product and one
to extract groundwater. Both double-
and triple-pump DPE systems extract the well
liquids separately from the soil vapor and are
similar in operation and application

Dual-pump DPE systems are simply a
combination of traditional soil vapor
extraction (SVE) and groundwater (and/or
floating product) recovery systems.
Dual-pump systems tend to be more flexible than
single-pump systems, making dual-pump
systems easier to apply over a wider range of
site conditions (e.g., fluctuating water tables,
wide permeability ranges); however,
equipment costs are higher.

Advantages:

Proven performance over a wide range of conditions. Requires no downhole pumps, but is flexible enough to allow their use if necessary.

Minimal disturbance to site operations; can be used under buildings without excavation.

Short treatment times (usually 6 months to 2 years under optimal conditions).

Substantially increases groundwater extraction rates.

Can be applied at sites with free product, and can be combined with other technologies.

Can reduce the cost of groundwater treatment through air stripping within the vacuum extraction tube.

Disadvantages:

Single-pump systems are expensive to implement at sites with medium to high-permeability soils, and effectiveness. Dual-pump systems may not be effective in low permeability soils.

Difficult to apply to sites where the water table fluctuates unless water table depression pumps are employed.

Treatment may be expensive for extracted vapors and for oil-water separation.